: The function of the basal ganglia in the initiation and elaboration of movement depends upon the generation of properly timed episodes of action potential generation by the projection neurons of the neostriatum. The goal of these studies is to discover the anatomical and neurophysiological mechanisms that determine whether or not such an episode will occur in an individual neostriatial neuron, and the timing of these episodes when they do occur. Anatomical experiments will employ in vivo intracellular biocytin staining of the cortical neurons that project to the neostriatum, to visualize single axons from this largest and most important of the inputs to the basal ganglia. The experiments will allow specific identification of neurons in the cerebral cortex that project to the strisomes, matrisomes, and extended matrix of the neostriatum. The boundaries between striosome and matrix will be visualized using immunocytochemistry for calbindin-D-28k, which specifically labels the matix. Quantitative studies of their axonal arborization will reveal the degree to which single axonal avbors may innervate individual neostriatal spiny neurons, the degree to which nearby neostiatal neurons receive similar inputs, and the spatial organization of converging cortical inputs to neostriatal projection cells. Electron microscopy will be used to insure that the light microscopic analyses of axonal varicosities are accurate estimates of the distribution of synaptic contacts. Physiological experiments will employ in vivo intracellular recording of striatal spiny neurons during selective stimulation of corticostriatal inputs to the stiosmal, matrisomal, and extended matrix stained compartment. The spiny cell studied will be stained intracellularly and their axons traced to determine whether they participate in the direct or indirect pathway. These experiments will allow a test of the idea that neurons of the indirect pathways receive separated and specific corticostriatal inputs.